In less than a decade, Francois Mitterrand has transformed his Socialist Party from a dying organization into one that, win or lose, seems bound to emerge from his weekend's legislative elections as the strongest political force in France.

"I used to vote Socialist with death in my heart," said Pierre Joxe, one of the party's rising young leaders. "But, now, things are completely different. This party has no relationship to the old one."

Joxe, 43, thpifies the kind of new blood that has transformed the party. A product of France's best schools, he was on the barricades in the Latin Quarter of Paris in the worker-student revolt of May 1968 while his father was a member of President Charles de Gaulle's cabinet.

Before Mitterrand took over the party it had fallen so low that in 1969 the Socialist presidential candidate, Gaston Defferre, polled 5 percent of the vote. Now, the polls show the party heading toward 30 percent of the vote in the first round of the elections Sunday.

What made possible the renaissance of the Socialist Party was a pledge, formally taken with Mitterrands's urging at its founding congress in 1971, never again to indulge the old party's platforms and allies and governing habit of getting elected with leftist with conservative ones.

In addition to Mitterrand's generally moderate personal following that joined with the Socialist old guard to transform the party, the new Socialist leader brought in active militants of the generation of young people like Joxe who almost overthrew De Gaulle in 1968 with a revolutionary street carnival.

Mitterrand wrested control of the party from former Premier Guy Mollet, who was elected in 1956 to head a leftist government and wound up fighting the Algerian war.

Mollet typified the heavily American-backed Third Force strategy of trying to govern France with everyone between the Communists on the left and Gaullists on the right. The internal strains among the rulers were so severe that the only way to deal with them was the revolving-door Cabinets of the Fourth Republic.

Mitterrand was highly identified with that period. He held his first Cabinet post at 28 and went on to hold 10 others.

"What makes us different today," said Charles Hernu, one of Mitterrand's closest personal and political friends, "is that we want nothing to do with Molletisme."

That is partly why Mitterrand could call on President Carter in Paris but has consistently spurned French President Valery Giscard D'estaing's overtures. If Mitterrand were to go to the Elysee Palace, a large part of his following would be very likely suspect him of preparing political treason.

The triumphant conservative backlash against the revolt of 1968 gave the impression that it had evaporated without a trace. But it has opened the floodgates to new currents of Socialist thinking.

Mitterrand managed to rally most of the organization-minded young Socialists scattered in a multiplicity of factions.

Mitterrand was perhaps the only man who would do it because he brought the Communists out of the political ghetto they entered in 1947 when they were thrown out of the Cabinet by a Socialist premier. Holding consistenly to leftist unity since 1965, Mitterrand generally has erased his old image as a political opportunist.

Under Mitterrand's leadership, party membership has gone from a low of 75,000 to 185,000 at last count, and it is still growing. The young Socialist of the 1968 generation have largely grouped in opposition to Mitterrand inside the party. The acronym of their group's name, CERES, stands for the Roman goddess of fertility.

CERES represents a quarter of the total party membership and is the party's cutting edge that is in direct competition with the Communists. It leads the way in increasing party membership and voting strength in France's most traditionalist, Gaullist and Catholic areas - Brittany, Alsace, Champagne and inside the Paris city limits.

The new recruits are both Catholic and Marxist. Sometimes the same person is both, a combination that fewer Frenchmen view as heretical these days. By dropping its sectarian hostility to Catholicism, the new Socialist Party has been able to reach out to groups that leftists used to reject automatically.

Speaking in an office in the National Assembly with a portrait of Karl Marx on the wall, Jerome Clement, the right-hand man of CERES leader Jean-Pierre Chevenement, said of Mitterrand:

"We judge people by their actions. We don't have absolute confidence in him. But we have no reason to accuse him of treason."

For Clement, CERES stands for four main ideas that are accepted in verying degress by the rest of the party.

Unity of the left. Clement complains that Mitterrand's concept of unity with the Communists is "clasping hands to hand-wrestle."

A break with capitalism. The party, CERES charges, includes a conservative fringe symbolized by such Socialist old-timers. As Mayor Defferre of Marseilles and by such "technocrats" as economist Michel Rocard, who are tempted by Giscard's siren call to modernize capitalism, instead of trying to find a French path to socialism in alliance with the Communists.

National independence. Gen. deGaulle's legacy of strict military independence from the United States has been enthusiastically adopted by French leftists as a way out of the need to make a choice between Washington and Moscow.

Worker self-management in factories and businesses. The growing influence of this idea in France as the key to an original French path to socialism is perhaps the most generally accepted and lasting heritage of the 1968 uprising.

There is an important wing of the party that is unmoved by the kind of revolutionary romanticism represented by CERES. That wing consists of two groups - the old guard and the new technocrats.

The old guard is still the most powerful segment of th party: Just between them, Defferre of Marseilles and Pierre Mauroy, the mayor of Lille and officially Mitterrand's deputy, control a quarter of the party membership with the regional party organizations they lead, not to mention their allies elsewhere in the country.